Method and apparatus for controlling color of a printing press based upon colorimetric density differences

Abstract

When a color-difference exceeds an established color tolerance, spectral reflectance values of a test area are converted to colorimetric densities and compared with target colorimetric density values for the corresponding test area from which a colorimetric density difference is established. The colorimetric density difference is utilized to determine an ink correction value.

What is claimed is:  
     
       1. A method of controlling color on a printed sheet at a printing press, the method comprising: 
       measuring the spectral reflectance values of a test area on the printed sheet;  
       converting the measured spectral reflectance values to test color coordinates in a first uniform color space;  
       comparing the test color coordinates in the first uniform color space to target color coordinates in the first uniform color space to obtain color parameter difference values in the first uniform color space;  
       comparing the color parameter difference values to established color tolerance values to determine whether the color parameter difference values exceed a color tolerance threshold value;  
       if the color parameter difference values exceed the color tolerance threshold value, then calculating a control signal value for the printing press using control signal difference values resulting from a control difference value that is based upon a comparison of test color coordinate values measured in the test area on the printed sheet in a second uniform color space that is different than the first uniform color space and target color coordinate values in the second uniform color space.  
     
     
       2. A method as in  claim 1 , and wherein the control signal value for the printing press is calculated by converting the control difference value to solid ink density differences using a linear matrix equation. 
     
     
       3. A method as in  claim 1 , and wherein the first uniform color space is CIELAB. 
     
     
       4. A method as in  claim 1 , and wherein the first uniform color space is CIELUV. 
     
     
       5. A method as in  claim 1 , wherein the second uniform color space is based upon colorimetric density values.